This invention relates generally to directional drilling of boreholes in the earth, and more particularly concerns improvements in such drilling techniques and apparatus employing mud motors.
Early apparatus and methods employed a device known as a whipstock, that was lowered into a borehole and oriented to the direction of desired borehole divergence from its initial path. That apparatus had a tapered portion that would force the drill bit to diverge in the oriented direction. Later apparatus and methods were developed that used a down-hole motor, driven by drilling-mud flow or other means. Such motors may be mounted to the lower end of a bent subassembly, such that the longitudinal axis of the motor and the drilling bit at its lower end, are at a slight angle to the direction of the drill string above the bent subassembly. When it is desired to drill in a generally straight path, the motor is activated and the drill string is continuously rotated. When it is desired to cause the path of the borehole to diverge in a given direction, continuous rotation of the drill string is stopped. The drill string, bent subassembly, motor and bit are then rotated to position the direction of bend in the bent subassembly in the desired direction of divergence, the upper part of the drill string is held in this position, and the down-hole motor is started. This causes the subsequent borehole to diverge in the desired and selected direction. More commonly, in current practice, sufficient flexibility or allowable angular motion between the motor and the bit is relied upon to permit alternative placements of the bent subassembly between the motor and the bit.
With respect to each of these alternative placements of the bent subassembly, the radius of curvature of the borehole path away from its normally straight orientation and the related angular deviation per unit of along-hole drill progression depend directly on the bend angle of the bent subassembly. Further, when trying to drill a straight hole by rotating the drill string as well as the motor connected to the drill bit, a bent subassembly tends to cause the borehole diameter to be larger than it would be in normal drilling. In the early usage of such processes and equipment, it was necessary to pull the drill string and change the bent subassembly to one of a different bend angle to achieve a change in the angle change per unit of along-hole drill progression and the related radius of curvature. Also, if long straight sections of hole were to be drilled, it was often best to pull the drill string to remove the bent subassembly. Either or each of these procedures caused delay, and therefore increased cost in directional drilling operations.
One expedient was to provide a bent subassembly that would enable its bend angle to change, as desired as by commands from the surface that were transmitted downwardly in the borehole to cause the desired change in bend angle. The very high stresses and very high axial mechanical loads in the drill string have generally led to very complex and costly mechanisms to achieve the desired capability to change the bend angle by surface command. Examples of the prior art include U.S. Pat. Nos. 4,077,657, 4,303,135, 4,394,881, 4,442,908, 4,745,982, 5,052,501, 5,117,927, 5,168,943, 5,343,966 and 5,479,995. Most of these do not provide means to control the bend angle when the drilling assembly is downhole. A few that do provide such control provide only limited control, for example no bend or a fixed bend angle. None of such prior art satisfies the need for simplicity, degree of control and prospective low cost for acquisition, operation and maintenance.
Of some interest are recent prior art mechanisms characterized by a two-axis bent subassembly that can provide bend angles in two nominally orthogonal directions at a very rapid rate. These devices are generally not used with a downhole motor to drive the rotation of the drill bit. These mechanisms are generally used in certain steerable rotary drilling approaches that steer the borehole path while continually rotating the drill string and manipulating the bend angle synchronously with this drill string rotation, the resulting direction of deviation of the borehole being determined in effect by the relative phase or phases of the bend angle motions and the drill string rotation angle. In such mechanisms, the control bandwidth must be at least equal to the maximum drill string rotation rate of usage. Examples of the prior art of this type include some early U.S. Patents such as U.S. Pat. Nos. 3,743,034 and 3,825,051 and more recent examples such as U.S. Pat. Nos. 6,296,066, 6,598,687, 6,607,044, 6,843,332 and 7,195,083. These are not relevant to the single-angle bend mechanism of the present invention. They require much more complex mechanisms requiring high-speed angular motions at the drill string rotation rate in two orthogonal directions.